1. Field of the Invention
The present invention relates to a delay line, and more particularly, to a delay line for use in delaying a signal such as a digital signal.
2. Description of the Related Art
Conventionally, as a delay line for use in delaying a signal such as a digital signal having a clock frequency of 100 MHz or higher, a lumped constant type LC low pass filter that is constructed by combining an inductor with a capacitor, has been used. Then, by increasing the inductance value and the capacitance value of such an LC low pass filter, it is possible to obtain a longer delay time.
However, with the above-described conventional delay line, by merely increasing the number of windings when forming an inductor in order to increase its inductance value, or merely increasing the areas of capacitive electrodes of a capacitor in order to increase its capacitance value, there will occur a problem that the frequency characteristic (a pulse rise characteristic) of the delay line will be worsened, or a problem that the flatness of a group delay time (waveform deflection characteristic) will be deteriorated.
In order to overcome the problems described above, preferred embodiments of the present invention provide a compact, very small delay line having an excellent flatness in its group delay time characteristic, and also having an excellent frequency characteristic.
According to one preferred embodiment of the present invention, a delay line includes a coil divided into at least three inductors, and at least three stages of low pass filters including the at least three inductors and a plurality of capacitors, wherein the low pass filters are contained within a laminated body defined by laminating a plurality of insulating layers.
With the above constitution, since each of the inductors and capacitors has only a small inductance value and a small capacitance value, respectively, there is little possibility that a pulse rise characteristic and a flatness of a group delay time of the delay line will be deteriorated. On the other hand, since the delay line as a whole has a large inductance value and a large capacitance value, the delay line achieves a long delay time.
Further, since coil conductor patterns constituting the inductors in each stage of the low pass filters are arranged on the same plane of insulating layers of the laminated body, it is possible to more efficiently arrange the coil conductor patterns within the laminated body.
In the delay line according to preferred embodiments of the present invention, each inductor has a coil axis that is substantially parallel to a laminating direction of the insulating layers, and winding directions of adjacent inductors are opposite to each other.
Moreover, since the insulating layers are preferably made of a dielectric ceramic material having a relative dielectric constant of about 15 or less, a magnetic coupling and an electrostatic coupling between mutually adjacent inductors are greatly reduced. As a result, an undesired surge or increase in a group delay time characteristic is prevented, thus ensuring a flat group delay time characteristic.
In addition, a capacitor connected to an end of at least one of the inductors constituting the low pass filters, and another capacitor connected to another end of the at least one of the inductors, are arranged in different positions in the laminating direction of the insulating layers. In this way, the low pass filter including the inductor and the two capacitors has only a small electrostatic capacitance coupling between the two capacitors, thereby obtaining an even flatter characteristic of a group delay time.
Further, in the delay line according to preferred embodiments of the present invention, a ratio of a vertical dimension to a lateral dimension of the coil conductor pattern is preferably approximately 1. When the coil conductor pattern has a substantially rectangular shape, the coil conductor pattern is defined as substantially a square shape, thereby making it possible to obtain a high Q.
Moreover, in the delay line according to preferred embodiments of the present invention, one end of an inductor of a k stage in the low pass filter and one end of an inductor of a k+1 stage adjacent thereto in the low pass filter are electrically connected to each other on an upper layer of the laminated body, and the other end of the inductor of the k+1 stage in the low pass filter and one end of an inductor of the k+2 stage adjacent thereto in the low pass filter are electrically connected to each other on a lower layer of the laminated body.
Adjacent inductors of adjacent low pass filters are alternately connected on the lower layer and the upper layer of the laminated body, thereby causing the magnetic fluxes of mutually adjacent inductors to be arranged in opposite directions. Due to this unique arrangement, magnetic coupling of the mutually adjacent inductors is greatly reduced, and cross-talk is minimized.